Power management method of and apparatus for use in a wireless local area network (LAN)

ABSTRACT

In a wireless network containing battery powered stations, one station is designated as a control point (CP). Each of the remaining stations has two operating modes: an active mode in which it receives and transmits messages, and an inactive or sleep mode which incurs reduced power consumption. The Control Point periodically transmits a beacon signal, and each station switches to its active mode to receive at least some of these beacon signals. A beacon signal may indicate that data is available for transmission to a particular station (from another station), in which case that particular station then broadcasts a message to indicate that it is awake and can receive messages. A beacon signal also indicates whether a broadcast period is in progress, and time to elapse until a next possible broadcast period will commence; if the Control Point has previously received and stored any broadcast messages it will re-broadcast them during the next following broadcast period and the remaining stations will react by remaining in active mode for the duration of that period.

FIELD OF THE INVENTION

[0001] This invention relates to wireless communications. It provides amethod by which data can be delivered to stations that are participatingin a wireless network but which are using a power management strategyand therefore are periodically in a low power mode and cannot transmitor receive data.

BACKGROUND

[0002] One of the main advantages of a wireless network is that itenables communication between mobile battery powered devices (stations)and also communication between these devices and devices connected toother communications networks. One of the critical factors for suchmobile devices is to enable them to operate for a long period of time.In designing such a device a designer aims to reduce the overall powerconsumption and therefore, increase either the operating time of thedevice, or reduce the number and size of the batteries required to powerthe device and thus make the device smaller.

[0003] In such a battery powered device each of the subsystems has aparticular power requirement. Some of these subsystems have higher powerrequirements than others and in order to increase the overall operatingtime of such devices it would be advantageous to be able to switch offsome subsystems or operate them in a low power mode until they arerequired. All wireless devices use power when they transmit or receivedata (and in most cases will consume significantly more power intransmit mode than in receive mode). It is therefore very advantageousto minimise the time when such a device has to be fully powered up (inan awake state) to send or receive data and maximise the time when itcan be in a powered down state (asleep).

[0004] A station which implements power saving functions can be in oneof three states:

[0005] In Power Saving (PS) mode and awake (or active);

[0006] In Power Saving (PS) mode and asleep or inactive;

[0007] Non-Power Saving mode.

[0008] The invention provides a means by which unicast data messages canbe delivered to a station operating in a power saving mode; it alsoprovides a means by which broadcast messages (including multicastmessages) can be delivered to a station operating in a power savingmode.

SUMMARY OF THE INVENTION

[0009] According to one aspect of this invention there is provided amethod of operating a wireless network containing a plurality ofstations, one station being designated as a control station and theremaining stations having an active state and an inactive state whichincurs lower power consumption than the active state, wherein:

[0010] the control station periodically transmits a beacon signal;

[0011] each remaining station periodically enters its active state toreceive the beacon signal; and

[0012] if there are data for a remaining station to receive, thatstation transmits a status signal to indicate to other remainingstations that it is active and able to receive data.

[0013] In one embodiment of this first aspect, a station (A) that wantsto send data in a unicast message to a station (B) in power saving (PS)mode sends a Power Management Service Request message to a Control Point(CP). The CP transmits a Control Point Beacon (CPB) at regularintervals, in which it includes a wake-up command for the station B inpower saving mode. Station B periodically wakes up and receives the CPB.If the CPB contains a wake-up flag for station B then station Bbroadcasts a Power Management Status message, containing its identity,to indicate that it is awake. When station A receives the PowerManagement Status message it can start sending data to station B. The PSstation goes back to sleep when it has not received or sent any unicastdata messages for a specified time, the PowerMgtTimeout period, and thelast CPB it received did not have its wake-up flag set.

[0014] According to another aspect of this invention there is provided amethod of operating a wireless network containing a plurality ofstations, one station being designated as a control station and theremaining stations having an active state and an inactive state whichincurs lower power consumption than the active state, wherein:

[0015] the control station periodically transmits a beacon signalcontaining a flag value indicating whether a broadcast period is inprogress and a count value indicating time to elapse until commencementof a next possible broadcast period;

[0016] each remaining station periodically enters its active state toreceive the beacon signal;

[0017] each remaining station remains in its active state if thebroadcast flag value indicates a broadcast period is in progress, andotherwise returns to its inactive state; and

[0018] the control station stores broadcast messages (includingmulticast messages) transmitted by any remaining station outside abroadcast period, and broadcasts such stored messages during asubsequent broadcast period.

[0019] In one implementation of this second aspect, the CP at regularintervals transmits a CPB in which there is a counter that identifieswhen a broadcast period will commence. All stations in power saving modewill wake up at the start of this broadcast period to receive the CPB.The CPB also contains a broadcast flag. If the broadcast flag is set thePS stations remain awake and only go back to sleep when they receive aCPB in which the broadcast flag is clear. Stations with broadcastmessages to send do not wait until this broadcast period starts, butsend them as appropriate. The Control Point receives and stores suchbroadcast messages which are transmitted outside the broadcast period.If at the start of the broadcast period the CP has any broadcastmessages stored, it will set the broadcast flag in the CPB andre-broadcast the stored messages during the broadcast period. Thebroadcast flag in the CPB remains set until the CP has re-broadcast allthe broadcast messages it has stored.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Methods and apparatus in accordance with the invention will nowbe described, by way of example, with reference to the accompanyingdrawings, in which:

[0021]FIG. 1 shows a wireless network system in which the inventioncould be used;

[0022]FIG. 2 shows a protocol for transmitting unicast data to a powersaving station;

[0023]FIG. 3 is a timing diagram showing a broadcast period in relationto a dwell period;

[0024]FIG. 4 shows a protocol for receiving broadcast messages; and

[0025]FIG. 5 is a state transition diagram for a power saving station inthe network.

DETAILED DESCRIPTION

[0026] Referring to FIG. 1, the invention is applicable to a networkwhich consists of a Control Point (CP) and a number of stations whichmay be battery powered and employ wireless transceivers in order totransmit and receive information. An example of such a network isdescribed in the IEEE 802.11 standard. The Control Point may or may notbe connected to another network.

[0027] Each station in the network has a identifier or address which isunique within all the stations, including the Control Point, which areparticipating in the network. The operation of each station iscoordinated by, for example, a processor within it operating inaccordance with software or firmware instructions stored in associatedmemory and implementing the steps constituting the power managementscheme protocol.

[0028] In the scheme described time is divided into a sequence of fixedlength periods or frames. In this description these periods are calleddwell periods.

[0029] For correct operation the power management schemes describedherein require the periodic transmission of a Beacon by a Control Point.They do not require each dwell period to be split into a sequence ofsmaller periods in which the transmission of data is under the controlof the Control Point.

[0030] Stations using power saving notify the Control Point. If thestation switches between the Active and Power Saving modes of operation(or vice-versa) it broadcasts a Power Management Status message, with aflag, the Power Saving Mode flag, set appropriately.

[0031] The following sections describe one possible implementation ofthe control messages required to operate the power management scheme.

Control Point Beacon

[0032] The Control Point Beacon (CPB) is broadcast by the CP at thestart of every dwell period. The CPB can include a number of sets ofinformation including an optional set of power management information.The power management information is only present if there is at leastone station in the network using power management services.

[0033] The Power Management information consists of: a length fieldwhich is used to determine the number of sets of station informationincluded in the CPB; a broadcast data field; a sequence of stationinformation fields containing the address of an individual station; andan event field for that station which contains a set of control andinformation flags. Station 1 Station N Length in octets BroadcastStation Event Station Event of power manage- data field Address FlagsAddress Flags ment information

[0034] The broadcast data field contains the following fields: BitPosition Description 0-6 Broadcast countdown counter. Indicates thenumber of dwell periods before the next broadcast period. A value of 1indicates that the next dwell period is the start of the broadcastperiod. 7 Broadcast flag - Set if this dwell period is part of thebroadcast period.

[0035] The Event Flags field can contain the following flags: Flag NameDescription Power Saving Set: Registration as a PS station wasRegistration Flag successful. Station can now use PS capabilities. PowerSaving Set: Power Management Service Request Denial Flag messagereceived and PS services denied by the CP but, acknowledges that a PowerManagement Service End message was received from the station. Wake-upFlag Set: When it receives the CPB the addressed station should come outof power-save mode, broadcast a Power Management Status message andremain awake. Clear: No action required. Wake-up Request Set: A PowerManagement Service Request Acceptance Flag message containing a wake-uprequest was received from the station and was accepted. Wake-up RequestSet: A Power Management Service Request Denial Flag message containing awake-up request was received from the station but was rejected.

[0036] Power Management Service Request

[0037] The Power Management Service Request message is used by a PSstation to request power management services from a Control Point. Themessage is also used by a station with data to send to a PS station torequest the Control Point to wake-up the PS station.

[0038] The format of the message is: Request Address of requestingAddress of the PS Field station station Optional

[0039] The Request Field contains two flags which identify the servicesthat are being requested from the CP. Flag Name Description PowerManagement Set: Station is requesting Power Management Service Requestservices. The optional field, Address of PS Station, is not used.Wake-up PS Station Set: Requests the CP to wake-up a PS station.Request. The optional field contains the address of the station to bewoken up.

[0040] The Control Point will respond to a Power Management ServiceRequest message in the Control Point Beacon. If a station transmits aPower Management Service Request message and does not receive anacknowledgement in the next Control Point Beacon, it should repeat thePower Management Service Request up to a predetermined number of timesor until an acknowledgement is received in the Control Point Beacon.

[0041] Power Management Service End

[0042] A station that no longer requires power management services sendsa Power Management Service End message to the CP. Once this message istransmitted by a station, the station can no longer depend on the CP forpower management services. The station must re-register with a CP beforeusing power management service again.

[0043] The format of the message is: Address of the sending PS station

[0044] Power Management Status Message

[0045] This message is broadcast by a PS station to notify otherstations on the network of its current power management state.

[0046] The message is also broadcast by a PS station after it hasreceived a Control Point Beacon indicating that it must wake-up from itslow power state. In this case the message tells other stations in thenetwork that the station is now awake and can receive unicast datamessages.

[0047] The format of the message is: Address of the PS station PowerSaving Mode flag

[0048] If the power saving mode flag is set it indicates that thestation is using power management and also that the station will remainactive for at least PowerMgtTimeout seconds after the transmission ofthe message. If the power saving mode flag is clear it indicates thatthe station is terminating the use of power management.

[0049] Delivery of Unicast Data Messages

[0050] Overview

[0051] Unicast data messages contain the address of the receivingstation and are intended only for the receiving station. Broadcastmessages contain a special address and are intended for reception by allstations participating in the network.

[0052] Requesting Power Management Support from a Control Point

[0053] A station which wishes to operate in power saving mode must be inthe Active state and remains in the Active state until it is providedwith power management support by a Control Point.

[0054] To request power management support from a CP the station sends aPower Management Service Request message to the CP.

[0055] The CP will respond to the Power Management Service Requestmessage using the power management section of the CP Beacon, notifyingthe station whether it can provide power management services.

[0056] If the CP can provide power management services the PS stationbroadcasts a Power Management Status message with the PS Mode flag setto notify all other stations that it is using power saving.

[0057] If the CP cannot provide power management services the stationhas the option of remaining in the Active state or going to sleep andrequesting power management services again at a later time.

[0058] Terminating Power Management Support from a Control Point

[0059] A PS station in PS mode can switch to either the Active state orswitch off completely. In both cases the PS station sends a PowerManagement Service End message to the CP to notify the CP that it nolonger requires power management services. The Power Management ServiceEnd message should be repeated a predetermined number of times or untila CPB is received acknowledging receipt of the message and terminationof the power management services for that station. The PS station atthis point broadcasts a Power Management Status message with the PS Modeflag clear to notify other stations that it is no longer using powersaving.

[0060] Low Power Operation

[0061] A PS station may go into low power mode under the followingsituations:

[0062] The station has not sent or received a unicast data message fromanother station for a period of time greater than PowerMgtTimeoutseconds.

[0063] After waking up and receiving a CPB in which the station'swake-up flag is not set, and the broadcast flag is clear.

[0064] Receiving Unicast DATA Messages

[0065] A PS station will wake-up periodically to receive the CP Beaconand will remain awake until it receives a CP Beacon. A PS station doesnot have to wake-up to receive all the Control Point Beacons; in orderto conserve power it may choose to wake-up and receive only one in everyn Beacons. The CPB wake-up flag associated with a station in PS modewill remain set for a number m of CPB transmissions.

[0066] If the wake-up flag for the station is set in the Beacon, the PSstation will remain awake and broadcast a Power Management Statusmessage. This message notifies any station that wishes to communicatewith the PS station that the PS station is awake and able to receivemessages.

[0067] If the CP Beacon does not include a wake-up flag for thisstation, then the station may immediately go back to low power mode.

[0068] The wake-up flag in the CP Beacon is cleared when the CP receivesa Power Management Status message from the power saving station.

[0069] Sending Unicast DATA Messages to a Power Saving Station

[0070] A station with a unicast data message to send to a PS stationfirst checks its own internal tables to determine the time when it lastsent a message to, or received a message from, the PS station. If thetime elapsed is less than PowerMgtTimeout seconds the station can assumethat the PS station is awake and send the unicast data message. If thetime elapsed is greater than PowerMgtTimeout seconds, then the stationmust send a Power Management Service Request to the CP requesting thatthe target station be awakened, i.e. unicast messages for a PS stationare buffered by the sender.

[0071] When the recipient of a unicast data message receives the messagecorrectly it transmits an acknowledgement message to the sender. If thesender does not receive an acknowledgement within a specified timeoutperiod the sender retransmits the data message.

[0072] The CP will indicate in the CPB whether the request has beenaccepted and if it was accepted that the CP will wake-up the PS station.If the request is accepted the sender waits for a Power ManagementStatus message from the PS station, and after receiving this messagesends the data.

[0073] It should be noted that each station has to keep track of whichstations are PS stations.

[0074] The protocol described above for transmitting Unicast data issummarized in FIG. 2. It has the advantage that a PS station can reduceits power consumption since it only needs to be awake periodically toreceive the Control Point Beacons. Once the station is awake itbroadcasts a Power Management Status message which notifies all stationsthat it is awake; it can then exchange data directly with one or morestations without requiring the data messages to be buffered andretransmitted by the Control Point, giving efficient utilisation of thescarce resource constituted by the wireless medium.

[0075] Delivery of Broadcast Data Messages

[0076] The Control Point is responsible for delivering broadcastmessages to PS stations during the broadcast period. No extrafunctionality is required in other stations (except PS Stations) tosupport broadcast or multicast messages. Since all multicast messagesare also sent to the broadcast address the remainder of this section andsubsequent sections will refer to broadcast data messages only.

[0077] The broadcast period is at least one dwell period and may beseveral dwell periods depending on the number of broadcast messages tobe transmitted.

[0078] The broadcast period is effective when the broadcast flag in theCP Beacon is set—see FIG. 3.

[0079] Control Point Operation

[0080] When there are PS stations present in the network the CP willcapture and buffer all broadcast messages including those it generatesitself and retransmit them during the broadcast period.

[0081] The Control Point Beacon includes a broadcast countdown counterwhich defines the number of dwell periods to the next broadcast period.The broadcast countdown counter is decremented by one before sending theCPB. A countdown value of one indicates that the next dwell period isthe start of a broadcast period. Before sending the first beacon to betransmitted in the broadcast period the CP will (re)set the broadcastcountdown counter to identify the start of the next broadcast period.

[0082] If the CP has any messages to broadcast then it will set thebroadcast flag in the CPB transmitted at the start of each dwell periodin the broadcast period. The broadcast period ends when the CP transmitsa beacon in which the broadcast flag is not set.

[0083] The CP will transmit any buffered broadcast messages in the dwellperiod using an appropriate medium access control mechanism. If at theend of the dwell period the CP has not broadcast all the messages it hasstored then it will extend the broadcast period by setting the broadcastflag in the CPB transmitted at the start of the next dwell period.

[0084] Any broadcast messages received by the Control Point during abroadcast period (i.e. in a dwell period in which the broadcast flag inthe CPB is set) do not need to be buffered and retransmitted by theControl Point since all stations are awake at this point and able toreceive the broadcast messages.

[0085] The CP determines the length of the broadcast period and the timeto the next broadcast period depending on the number of broadcastmessages on the network.

[0086] Receiving Broadcast Messages

[0087] To receive broadcast messages a PS station should notify theControl Point that it is going to be using power management (see sectionon Requesting Power Management Support from a Control Point) and thenremain awake until it receives a Control Point Beacon containing thebroadcast countdown counter. From the countdown counter the station canthen determine the start of the broadcast period and go into sleep modeuntil immediately before the start of the broadcast period when it willwake-up to receive the CPB at the start of the broadcast period.

[0088] If a PS station receives a CPB in which the broadcast flag is setit should remain awake to receive broadcast messages. The PS stationshould remain awake until it receives a CPB in which the broadcast flagis not set.

[0089] If a PS station receives a CPB in which the broadcast flag is notset it can immediately go to sleep until the start of the next broadcastperiod.

[0090] If a PS station has woken up for the broadcast period and doesnot receive a CPB, then it should remain awake until it receives a CPBeacon, at which point it can determine whether it should remain awakeor go back to sleep.

[0091] The protocol described above for receiving broadcast messages issummarized in FIG. 4 (Bcast cntr=broadcast countdown counter). It hasthe advantage that PS Stations only need to be awake during thebroadcast period, enabling them to save power. The broadcast period islonger than the time required for the Control Point to send its bufferedmessages; during this period any station can send broadcast messages andthere is no need for the Control Point to buffer and re-send thosemessages, improving utilisation of the wireless medium.

[0092]FIG. 5 is a state transition diagram summarising the operation ofa power saving station to receive unicast and broadcast messages asdescribed above.

1. A method of operating a wireless network containing a plurality ofstations, one station being designated as a control station and theremaining stations having an active state and an inactive state whichincurs lower power consumption than the active state, wherein: thecontrol station periodically transmits a beacon signal; each remainingstation periodically enters its active state to receive the beaconsignal; and if there are data for a remaining station to receive, thatstation transmits a status signal to indicate to other remainingstations that it is active and able to receive data.
 2. The method ofclaim 1, wherein the control station incorporates into its beacon signalan indication of the identity of each remaining station for which thereare data to be received by that station.
 3. The method of claim 1,wherein each remaining station returns to its inactive state when it hasnot received or transmitted any data during an interval of predeterminedduration.
 4. The method of claim 1, wherein each remaining stationperiodically enters its active state to receive every beacon signal. 5.A method of operating a wireless network containing a plurality ofstations, one station being designated as a control station and theremaining stations having an active state and an inactive state whichincurs lower power consumption than the active state, wherein: thecontrol station periodically transmits a beacon signal containing a flagvalue indicating whether a broadcast period is in progress and a countvalue indicating time to elapse until commencement of a next possiblebroadcast period; each remaining station periodically enters its activestate to receive the beacon signal; each remaining station remains inits active state if the broadcast flag value indicates a broadcastperiod is in progress, and otherwise returns to its inactive state; andthe control station stores broadcast messages transmitted by anyremaining station outside a broadcast period, and broadcasts such storedmessages during a subsequent broadcast period.
 6. The method of claim 5,wherein each remaining station periodically enters its active state toreceive every beacon signal.